Most electronic devices employ circuits that do not directly obtain operating power from mains AC electrical power. Instead, electronic devices often require one or more DC supply or bias voltages. In order to generate a regulated DC voltage derived from an AC power source, switching converters are often used. In one common embodiment of a switching converter power supply, an AC input voltage is rectified to a first DC voltage level, which is in turn converted to an output DC voltage level. A DC-DC converter circuit performs the function of converting the first DC voltage level to the output DC voltage level.
There exist a number of different DC-DC converter architectures. A typical DC-DC converter design employs a semiconductor switch in line with the input voltage that is turned on and off at a relatively high frequency (compared to the frequency of mains AC voltage). The high frequency switched signal passes through the primary winding of a transformer. The transformer is configured such that the output winding provides an output signal level that corresponds to the desired output DC voltage level. The output winding of the transformer is coupled to a rectifier circuit that converts the output signal to DC.
In order to regulate the output of the DC-DC converter, various feedback signals can be used. The feedback signal may be used by control circuitry to alter the operation of the high frequency switching device. By way of example, the output voltage may be used as a feedback signal. In general, if the feedback signal indicates that the output voltage is too low, presumably due to heavy loading, then the control circuitry may increase the duty cycle of the switched signal generated by switching device, which increases the power provided through the transformer to the output signal.
In some systems, multiple power supplies are used in parallel. Such systems can include modular equipment systems in which differing power needs are met by adding parallel supply modules. In particular, such modular equipment systems are typically designed such that different interchangeable modules may be combined in a variety of ways. The power consumption of the system depends on which modules are used in the system. Some configurations may require only one power supply while others may require multiple supplies.
One problem that arises with the use of parallel power supplies arises from differences in output voltage levels of the supplies. While parallel power supplies ideally have the same output voltage level, the output voltage levels in reality will typically differ slightly. When parallel supplies have different output levels, the supply with the higher output voltage typically provides the most current. In some cases, the imbalance of current between the two parallel supplies can be significant. This is undesirable because one supply will run at full load (hot and stressed) while the other will be nearly unused.
To avoid problems caused by heavily imbalanced current draw on parallel power supplies, it is known to provide a series resistor on each power supply output. While the resistor can help balance the current between the two supplies by increasing the output impedance, to make the supply look more like a current source rather than a voltage source, the output resistor consumes a significant amount of power, which leads to heat dissipation problems and inefficiencies.
Accordingly, there is a need for a power supply that is more amenable to being connected in parallel with a similar power supply that avoids problems due to current imbalance.